When and how did the copula originate?

Does present form "is" have anything to do with the English "is"?

Yes it has.

"Is" was already "is" in Old Irish, and it must have been *esti in Pre-Historic Irish. It is the same Indo-European root as you have in English is, German ist, French est, Latin est, Middle Welsh ys, etc.

For a full explanation see en.wikipedia.org/wiki/Indo-European_copula

Thanks, Lughaidh.
David, is wikipedia really a good source?
I've heard so many dubious things about it.

I like wickypedia, so if its disreputable then I guess I shouldn't use it but I just might continue anyway because everything I've seen on there I've found true.

I know this iis a question that has been asked before but I'll ask it again. How do you know when to use "ta" and when to use "is". I've never read an answer to that question that made sense to me. I wonder if perhaps its just one of those things that you just have to memorize, remembering which sentences use what.
.....

Riona and Norwegiandame, Wikipedia can be altered by anyone so you have to use your brain to distinguish the wheat from the chaff, but many articles on there have clearly been written by knowledgeable people, including the one on Russian phonology that someone keeps trying to alter, and I keep altering back. The one on the Indo-European copula seems well written and has a few written sources at the bottom.

Riona, the distinction between tá and is could not be simpler. Do you know the distinction between ser and estar in Spanish. Estoy enfermo - I am ill (this is a temporary state, a comment on how I am now, not who I am). Soy mexicano - I am Mexican (this is simply a fact, it is who I am).

Tá mé go maith: I am well (this is not an intrinsic quality belonging to me, it is a temporary state)
Tá mé anseo: I am here (this is where I am now, but not a statement of who or what I am.)

Is fear maith é: he is a good man. This is more of an intrinsic quality. It is what the person actually is.
Is mé Máirtín: I am Martin (this is an essential identification; you are not saying he is happy, he is sad, he is cunning, he is clever, you are saying who or what he is.)

NOrmally the way it is explained is that the copula is used to link pronouns and nouns, in other words to say what a thing is, rather than to comment on where it is or what it is like.

Tá is used with adjectives. The parallel with ser/estar in Spanish is not exact, by the way. Tá an leabhar spéisiúl. (contrast Spanish, el libro es interesante)

A Dhavid a chara,

That Spanish comparison helps a lot. I was hoping someone might be able to compare them because I understand ser and estar after studying Spanish at school, (sure and I'm done with that ... finally!). Go raibh maith agat.

Note: I have nothing against the language of Spanish. I just am so glad to be done with it. Of course my university doesn't have Irish so ...